We collected baseline data on organochlorine (OC) pesticide contamination in resident passerine birds from northwest Costa Rica in an area where pesticides had not been used in at least 30 years. Results were compared with a previous study of OC contamination in mayfly larvae (Euthyplocia hecuba) collected from the same region. Thirteen OC compounds were detected in the ng/g range for 19 of 56 birds sampled, and the highest OC frequencies were found in birds collected from Pitilla, the site closest to agricultural areas. Atmospheric transport could be a mechanism by which the pesticides are traveling from agricultural areas to areas where pesticides have never been used. OC levels were lower in birds than in mayfly larvae, which suggests that either the birds were not in the same food chain as the mayflies, or that the birds may have been younger that the larvae collected. Moreover, the OC contamination of the birds was dominated by p,p'-DDE, in contrast endosulfan dominated the mayfly OC contamination.



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